The refined protocol of MAPREC test for neurovirulent revertants in type 3 OPV was developed, and the reference samples prepared for the in-house test. The in-house test was performed on a variety of vaccine lots from different sources, with statistical evaluation of variability of data, and vaccine acceptability criteria formulated. This concludes the development of a molecular test for type 3 OPV. International Collaborative Study on MAPREC for type 3 OPV was designed and initiated in cooperation with WHO. Necessary materials and test samples were prepared and distributed to the Study participants together with manuals, protocol and report forms. Two foreign participants of the Study were trained by the lab personnel. In order to study the significance of known neurovirulent reversions at nucleotides 480 and 525 of type 1, and nucleotide 481 of type 2, MAPREC assays were developed, and extensive study of vaccine lots made by different manufacturers performed. This information will be used in determining the maximum permissible level of these revertants in vaccine lots and in monitoring the consistency of vaccine manufacturing. Genetic stability of type 1 and type 2 OPV was studied by serially passaging the viruses in cell culture substrates and subsequent analysis by a sequence heterogeneity assay. Several new consistently occurring reversions were identified. Quantitative MAPREC assays for these reversions are being developed. A potential role of these reversions in monkey neurovirulence and vaccine quality control is being studied.